make mkdir crash-safer, as noticed by many students on midterm

f3e87bc8 · kolya · e1626709 · f3e87bc8
--- a/sysfile.c
+++ b/sysfile.c
@@ -237,13 +237,6 @@ create(char *path, int canexist, short type, short major, short minor)
  ip->minor = minor;
  ip->nlink = 1;
  iupdate(ip);
-  
-  if(dirlink(dp, name, ip->inum) < 0){
-    ip->nlink = 0;
-    iunlockput(ip);
-    iunlockput(dp);
-    return 0;
-  }

  if(type == T_DIR){  // Create . and .. entries.
    dp->nlink++;  // for ".."
@@ -252,6 +245,17 @@ create(char *path, int canexist, short type, short major, short minor)
    if(dirlink(ip, ".", ip->inum) < 0 || dirlink(ip, "..", dp->inum) < 0)
      panic("create dots");
  }
+
+  if(dirlink(dp, name, ip->inum) < 0){
+    dp->nlink--;
+    iupdate(dp);
+    iunlockput(dp);
+
+    ip->nlink = 0;
+    iunlockput(ip);
+    return 0;
+  }
+
  iunlockput(dp);
  return ip;
 }